With the drastic increase of human population, there is an ever-growing demand for energy and clean water for the continuous economic growth and suitable inhabitation on earth. Over the years, federal government has applied distinct strategies to address these two needs separately: the municipal wastewater is collected by local wastewater plants for purification and subsequent reuse as reclaimed water, while the energy source is mainly based on natural gas, and crude oil. Apparently, these two strategies are decoupled. Millions tons of wastewater is produced from industrial and agricultural operations each year, and about 25 billion US dollars are spent annually for wastewater treatment in the United States alone.2 Meanwhile, the use of natural gas/petroleum generates a lot of greenhouse gas and toxic chemicals, which poses a serious threat to the environment, and also leads to additional cost to treat the pollution.
FIG. 1 shows a schematic drawing integrating photocatalysis with microbial metabolism to remediate wastewater and produce chemical fuels. The wastewater treatment and energy recovery can simultaneously be achieved by microbial fuel cell (MFC) technology. For instance, microbial electrohydrogenesis process has been experimentally demonstrated in microbial electrolysis cell (MEC) using a wide range of microorganisms with various organic nutrients to produced hydrogen. However, thermodynamic constraints limit microbial electrogensiss and hydrogen production occur simultaneously without the addition of an external bias. To overcome the thermodynamic constrains, an external bias is usually applied to sustain the current/hydrogen generation. Nevertheless, the need of external bias reduces the overall energy recovery ratio and adds to the complexity and cost for hydrogen production, making microbial electro-hydrogenesis less attractive as an energy solution. Considerable efforts have been made on optimization of MEC reactors, design of anodes, and catalysts to reduce the above-mentioned energy losses. Alternatively, to obtain the required energy from a renewable energy source is also a promising approach that can fundamentally address the issue.
Previously reported is a dye-sensitized solar cell (DSSC)-powered microbial electrolysis cell (MEC). The MEC was a conventional dual chamber device with the anode inoculated with anaerobic digester sludge from a sewage treatment plant and acetate was fed as the electron donor. The MEC was integrated with a conventional DSSC device composed of a ruthenium dye-loaded TiO2 nanoparticle film as working electrode and a platinized FTO glass as counter electrode. The DSSC device harvests sunlight to provide the required energy for hydrogen production. However, Ru is a rare and expensive element, which renders this approach to be unsustainable.
A prior art hybrid device is shown that includes a photoelectrochemical cell (PEC) device and a MFC device. Significantly, this hybrid device generates hydrogen gas at zero external bias using biodegradable organic matters and sunlight as the only energy sources. Shown in FIG. 2A, is a prior art PEC device composed of a TiO2 photoanode and a Pt cathode. The MFC is an air-cathode dual-chamber device, inoculated with either Shewanella oneidensis MR-1 (batch-fed on artificial growth medium) or natural microbial communities (batch-fed on local municipal wastewater). Under light illumination, the TiO2 photoanode provides a photovoltage of ˜0.7 V that overcomes the thermodynamic barrier for microbial electrohydrogenesis. As a result, a pronounced current generation and sustainable production of hydrogen gas (FIG. 2B). This hybrid device (with wastewater as anolyte) achieved only a decent solar conversion efficiency of ˜1% at zero external bias under one sun illumination, and only fair soluble chemical oxygen demand (SCOD) removal rate of ˜200 mg/L/day, which is comparable to the efficiency of some conventional microbial devices. The originally black wastewater can eventually turn into almost clear solution. Taken together, this hybrid device holds great promise of being set up in remote/rural areas, without electricity and fuel supplies, for self-sustained wastewater treatment and chemical fuel production.
A solar-assisted microbial device has been successfully demonstrated by the inventors. For instance, the hybrid MFC-PEC device achieved the overall solar-to-hydrogen conversion efficiency of ˜1%, which is very promising given that the device was operated in a sustainable manner using sunlight and wastewater as the only energy sources. What is needed is improvement in the performance of an MPS by enhancing the charge generation and collection processes.
There is urgent need to employ energy-efficient processes for wastewater treatment, and simultaneously recover the “wasted energy” contained as organic matters in wastewater.